Quantum tunneling photodetector array including electrode nano wires

ABSTRACT

A quantum tunneling photodetector array and a method of generating an image. The photodetector array comprises an array of pairs of opposing first and second electrodes; a photo-sensitive insulating material disposed between the opposing first and second electrodes of the respective pairs; an electrical circuit for detecting photo-assisted quantum tunneling currents between the opposing first and second electrode of the respective pairs.

FIELD OF INVENTION

The present invention relates broadly to quantum tunnelingphotodectector array and to a method of generating an image.

BACKGROUND

The demand for image sensors such as charge coupled device (CCD) andcomplimentary metal-oxide-semiconductor (CMOS) sensors has increasedsignificantly in recent years, primarily due to increasing demand inoptical devices such as camera cell phones and digital still and videocameras.

Furthermore, with the ongoing development of more and more consumerdevices which incorporate cameras, such as embedded cameras in personalcomputers (PCs) and especially in notebook PCs, or in automotive rearview cameras, together with the ongoing boom in camera phone andpersonal assistant (PA) devices, the demand for imaging sensors isexpected to continue to grow in the years to come.

It is with the knowledge of this background that the present inventionhas been made, and example embodiments of the present invention seek toprovide an alternative image sensor and a method of generating an imagein view of the ongoing and increasing demand for image sensors.

SUMMARY

In accordance with a first aspect of the present invention, there isprovided a quantum tunneling photodetector array comprising an array ofpairs of opposing first and second electrodes; a photo-sensitiveinsulating material disposed between the opposing first and secondelectrodes of the respective pairs; and electrical circuit for detectingphoto-assisted quantum tunneling currents between the opposing first andsecond electrode of the respective pairs.

A separation distance between the first and second electrodes may besubstantially fixed.

The opposing first and second electrodes may be supported on a firstsubstrate and a second substrate respectively.

The insulating material may be disposed between the first and secondsubstrates.

The insulating material may comprise a soft matter material.

The soft matter material may comprise one or more of a group consistingof thiols, self assembled monolayers (SAMs), organic solvents,perfluoropolyether PFPE, organic solvents, and polymeric brushes.

The insulating material may comprise a solid state material.

The solid state material may comprise one or more of a group consistingof a dielectric material, or a doped heterostructure double well barriersuch as Al_(x)Ga_(1-x)As/GaAs/Al_(x)Ga_(1-x)As.

At least one of the first and second substrates may be transparent.

The photodetector array may further comprise spacer elements maintainingthe substantially fixed separation distance and between the first andsecond electrodes.

The spacer elements may further maintain a parallelism of the pairs offirst and second electrodes.

The spacer elements may be formed as separate solid state materialspacers or are incorporated into the photo-sensitive material.

The array of opposing first and second electrodes may comprise across-grid of a first set of electrode wires and a second set ofelectrode wires, such that the first and second electrodes areconstituted as opposing sections of the electrode wires of the first andsecond sets at projected cross-points of the cross-grid.

The first and second sets of electrode wires may be disposed at about90° with respect to each other.

The electrical circuit may comprise individual electrical connections tothe electrode wires of the first and second sets.

The electrical circuit may further comprise a multiplexer component forde-multiplexing electrical signals from the first and second sets ofelectrode wires for detecting the quantum tunneling currents between theopposing sections of the electrode wires of the first and second sets.

The array of opposing first and second electrodes may be constituted asfirst and second sets of pixel electrodes respectively.

The electrical circuit may comprise individual electrical connections tothe pixel electrodes of the first and second sets.

The electrical circuit may further comprise a source for applying apotential difference across the first and second opposing electrodes.

In accordance with a second aspect of the present invention, there isprovided a method of generating an image comprising using a quantumtunneling photodetector array as defined in the first aspect.

In accordance with a third aspect of the present invention, there isprovided a method of generating an image comprising the steps ofdetecting photo-assisted quantum tunneling currents between opposingfirst and second electrodes of respective ones of an array of pairs ofthe opposing first and second electrodes; and generating the image basedon the respective detected photo-assisted tunneling currents between theopposing first and second electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readilyapparent to one of ordinary skill in the art from the following writtendescription, by way of example only, and in conjunction with thedrawings, in which:

FIG. 1 shows a schematic plane view diagram illustrating a quantumtunneling photodetector array according to an example embodiment.

FIG. 2 shows a detail of the quantum tunneling photodetector array ofFIG. 1.

FIG. 3 shows a schematic cross-sectional view of the quantum tunnelingphotodetector array of FIG. 1.

FIG. 4 shows a detail of FIG. 3, illustrating photo-assisted tunnelingin the quantum tunneling photodetector array of the example embodiment.

FIG. 5 shows an example plot of quantum tunneling current as a functionof potential difference, illustrating a comparison between measurementsin an ambient illumination and with fluorescent light illumination.

FIG. 6 shows a flow chart illustrating a method of generating an imageaccording to an example embodiment.

DETAILED DESCRIPTION

FIG. 1 shows a plane view of a quantum tunneling photodetector array 100according to an example embodiment. The array 100 comprises a first set102 of electrode wires here in the form of nano wires e.g. 104. A secondset 106 of electrode wires in the form of nano wires e.g. 108 isdisposed at about 90° with respect to the first set 102, thus forming across-grid of nano wires. The nano wires e.g. 104, 108 of the first andsecond sets 102, 106 respectively are disposed with a fixed separationdistance therebetween, i.e. the sets 102 and 106 are at a fixedseparation distance along an axis perpendicular to the plane of thedrawing in FIG. 1. Out of many possible fabrication processes for theformation of nano wires on e.g. carrier substrates, reference is made tofor example L-R. Hackett, Jr., Process and mask for ion beam etching offine patterns, U.S. Pat. No. 4,275,286 and J. R. Heath and M. A. Ratner,Molecular Electronics, Physics Today, May 2003, pp. 43-49 fordescription of example fabrication techniques suitable for use inexample embodiments of the present invention.

Electric circuit components 110, 112 are provided, and are electricallyinterconnected into individual ones of the nano wires e.g. 108, 104 ofthe second and first set 106, 102 respectively. The electrical circuitcomponents 110, 112 are controlled to apply a potential differencebetween the nano wires e.g. 104, 108 of the first and second sets 102,106, to read tunneling currents flowing between projected cross-overpoints of the grid, and to determine which projected cross-over pointwas struck by light as described below.

Turning now to FIG. 2, which shows a detail of the quantum tunnelingphotodetector array 100, it will be appreciated by a person skilled inart that the cross-grid of the first and second sets 102, 106 of nanowires constitutes an array of opposing first and second electrodes inthe formed of opposing sections e.g. 200 of the nano wire e.g. 202 and acorresponding opposing section (hidden in FIG. 2) of the nano wire 204at projected cross-points of the cross-grid.

The electrical circuit components 110, 112 are additionally configuredto function as a multiplexer for de-multiplexing electrical signals fromthe nano wires e.g. 202, 204 of the first and second sets 102, 106 fordetecting photo-assisted quantum tunneling currents between the opposingsections of the nano wires e.g. 202, 204 at the projected cross-pointsof the cross-grid. are understood in the art, and will not be describedherein in more detail. Out of many possible multiplexer components andtechniques for interrogating a network of sensing lines in a cross-gridarrangement, reference is made to for example P. J. Kuekes, R. S.Williams, Demultiplexer for a molecular wire crossbar network (MWCNDEMUX) U.S. Pat. No. 6,256,767 for description of an example multiplexercomponents and techniques suitable for use in example embodiments of thepresent invention.

Details of the operation of the example embodiment will now be describedwith reference to FIGS. 3 and 4. FIG. 3 shows a cross-sectional view ofthe quantum tunneling photodetector array 100 of the example embodiment.The first set 102 of nano wires e.g. 300 is embedded in a transparentsubstrate 302. In this example embodiment, the second set 106 of nanowires e.g. 304 is also embedded in a transparent substrate 306. However,it is noted that in different example embodiments, only one of the sets102, 106 of nano wires may be embedded in a transparent material, whilethe other set can be embedded in an opaque material substrate.

In the example embodiment, the transparent substrates 302, 306 cancomprise glass substrates, but it will be appreciated that differenttransparent materials can be used in different embodiments, including,but not limited to quartz, sapphire transparent oxides (TOX),semiconductor materials such as silicon or insulating or semiconductingoxides. The choice of material can also depend on the relevantwavelength of interest.

It is noted here that the example embodiment is not limited to operationin the visible wavelength range, but can be adapted to differentwavelength ranges, including an infrared (IR) wavelength range orultra-violet.

An insulating material 308 is disposed between the substrate 302, 306,in the fixed separation distance gap between the first and second sets102, 106 of nano wires. The insulating material 308 is chosen to havephoto-sensitive properties, more particular the material 308 is chosensuch that photo-assisted quantum tunneling can be achieved between thenano wires of the first and second sets 102, 106 at opposing sections ofthe nano wires e.g. 300, 304 at projected cross-points e.g. 310 of thecross-grid.

FIG. 4 shows a detail of FIG. 3. In operation, photons 400 entering thephoto-sensitive material 308 interact with the photo-sensitive material308 to achieve photo-assisted quantum tunneling 401 between opposingsections 402, 404 of nano wires 300, 304 of the first and second sets ofnano wires respectively. As described above with reference to FIG. 1,the electrical circuit components e.g. 110 are configured for applying apotential difference across the nano wires e.g. 300, 304 of the firstand second sets of nano wires, and for de-multiplexing electricalsignals from the nano wires e.g. 300, 304 of the first and second setsfor detecting the quantum tunneling currents between the opposingsections 402, 404 of the nano wires 300, 304.

FIG. 5 shows an example plot of quantum tunneling current as a functionof potential difference, illustrating a comparison between measurementsin an ambient illumination (curve 500) and with fluorescent lightillumination (502). As can be seen from a comparison of curves 500 and502, the effect of photo-assisted quantum tunneling is evidenced by theincrease in measured tunneling current at a given applied potentialdifference. Example embodiments of the present invention exploit thephoto-assisted quantum tunneling effect to sense photons or opticalsignals at the various projected cross-points of the cross-grid of nanowires for implementation of image sensors. As will be appreciated by aperson skilled in the art, e.g. threshold based signal processing, oranalog (spectral) signal processing can be performed in exampleembodiments, to generate an image detected by the quantum tunnelingphotodetector array. Threshold based signal processing and analog(spectral) signal processing are understood in the art, and will not bedescribed herein in more detailed.

Returning now to FIG. 4, the photo-sensitive material 308 in the exampleembodiment can comprise thiol. However, it will be appreciated thatdifferent photo-sensitive materials can be used in differentembodiments. Furthermore, the photo-sensitive material 308 may be a softmatter material, or a solid state material in different embodiments.Examples of suitable photo-sensitive materials include, but are notlimited to, thiols, self assembled monolayers (SAMs), organic solvents,perfluoropolyether (PFPE), or solid state materials such as a dopedheterostructure double well barrier such asAl_(x)Ga_(1-x)As/GaAs/Al_(x)Ga_(1-x)As or dielectric materials as usedin Resonant Tunneling Diodes (RTD), see for example H. Mizuta and T.Tanoue, The Physics and Applications of Resonant Tunneling Diodes,Cambridge U.P., 1995. The deposition of such photo-sensitive materialsis understood in the art, and will not be described herein in moredetail.

The fixed separation distance and two-dimensional parallelism betweensubstrates 302 and 306 with embedded nanowire arrays 102 and 106 ischosen such that tunneling current can flow between the projectednanowire cross-over points e.g 310. In embodiments in which thephoto-sensitive material 308 is a soft matter material, separate solidstate spacers e.g. 311 can be used to keep the separation distance fixedand the substrates 302, 306 parallel. Solid state, e.g. oxide, spacers311 can be fabricated on one or both substrates 302, 306, outside thenanowire arrays 102, 106 areas.

Alternatively, molecular species that can provide structural stiffnessmay be incorporated into the soft matter material. For example, carbonbucky-balls C₆₀ or other suitable species can be provided in solution inthe soft matter material.

In embodiments in which the photo-sensitive material 308 is a solidstate material, the separation distance can be kept fixed and thesubstrates 302, 306 can be kept parallel through the structuralintegrity of the solid state material itself, and/or through use ofseparate spacers e.g. 311 between the substrates 302 and 306.

FIG. 6 shows a flow chart 600 illustrating a method of generating animage according to an example embodiment. At step 602, photo-assistedquantum tunneling currents between opposing first and second electrodesof respective ones of an array of pairs of the opposing first and secondelectrodes are detected. At step 604, the image is generated based onthe respective detected photo-assisted tunneling currents between theopposing first and second electrodes.

The example embodiments described have applications as image sensors invarious optical devices, including, but not limited to, camera cellphones, camera PA devices, digital still cameras, digital video cameras,embedded cameras in PCs, automotive rear view cameras or night visioncameras. The example embodiments can provide image sensors withoperating voltages suitable for hand-held, battery operated devices, forexample in the range from about 0 to 25 volts, and with low powerconsumption, for example in the milli Watt range.

It will be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present invention asshown in the specific embodiments without departing from the spirit orscope of the invention as broadly described. The present embodimentsare, therefore, to be considered in all respects to be illustrative andnot restrictive.

For example, it will be appreciated that the sets of electrode wires inthe cross-grid may be disposed at different, non-zero angles withrespect to each other, other than the about 90° described for theexample embodiment. Furthermore, the present invention is not limited toimplementation with sets of electrode wires. Rather, in differentembodiments the array of pairs of opposing first and second electrodeelements can be implemented using two opposing sets of pixel electrodearrays. In such embodiments, the electrical circuit for detecting thephoto-assisted quantum tunneling current between the opposing first andsecond pixel electrodes comprises individual electrical connections tothe pixel electrodes, for pixel-ised image sensing similar to CCD orCMOS based image sensors.

The invention claimed is:
 1. A quantum tunneling photodetector arraycomprising: an array of pairs of opposing first and second electrodes,the array of opposing first and second electrodes comprising across-grid of a first set of electrode nano wires and a second set ofelectrode nano wires, such that the first and second electrodes areconstituted as opposing sections of the electrode nano wires of thefirst and second sets at projected cross-points of the cross-grid; aphoto-sensitive insulating material disposed between the opposing firstand second electrodes of the respective pairs; and an electrical circuitfor detecting photo-assisted quantum tunneling currents between theopposing first and second electrode of the respective pairs.
 2. Thephotodetector array as claimed in claim 1, wherein a separation distancebetween the first and second electrodes is substantially fixed.
 3. Thephotodetector array as claimed in claim 1, wherein the opposing firstand second electrodes are supported on a first substrate and a secondsubstrate respectively.
 4. The photodetector array as claimed in claim1, wherein the first and second sets of electrode wires are disposed atabout 90° with respect to each other.
 5. The photodetector array asclaimed in claim 1, wherein the electrical circuit comprises individualelectrical connections to the electrode wires of the first and secondsets.
 6. The photodetector array as claimed in claim 1, wherein theelectrical circuit further comprises a source for applying a potentialdifference across the first and second opposing electrodes.
 7. A methodof generating an image comprising using a quantum tunnelingphotodetector array as claimed in claim
 1. 8. The photodetector array asclaimed in claim 2, further comprising spacer elements maintaining thesubstantially fixed separation distance and between the first and secondelectrodes.
 9. The photodetector array as claimed in claim 3, whereinthe insulating material is disposed between the first and secondsubstrates.
 10. The photodetector array as claimed in claim 3, whereinat least one of the first and second substrates is transparent.
 11. Thephotodetector array as claimed in claim 9, wherein the insulatingmaterial comprises a soft matter material.
 12. The photodetector arrayas claimed in claim 9, wherein the insulating material comprises a solidstate material.
 13. The photodetector array as claimed in claim 11,wherein the soft matter material comprises one or more of a groupconsisting of thiols, self assembled monolayers (SAMs), organicsolvents, perfluoropolyether PFPE, organic solvents, and polymericbrushes.
 14. The photodetector array as claimed in claim 12, wherein thesolid state material comprises one or more of a group consisting of adielectric material, or a doped heterostructure double well harrier suchas Al_(x)Ga_(1-x)As/GaAs/Al_(x)Ga_(1-x)As.
 15. The photodetector arrayas claimed in claim 8, wherein the spacer elements further maintain aparallelism of the pairs of first and second electrodes.
 16. Thephotodetector array as claimed in claim 8, wherein the spacer elementsare formed as separate solid state material spacers or are incorporatedinto the photo-sensitive material.
 17. The photodetector array asclaimed in claim 5, wherein the electrical circuit further comprises amultiplexer component for de-multiplexing electrical signals from thefirst and second sets of electrode wires for detecting the quantumtunneling currents between the opposing sections of the electrode wiresof the first and second sets.
 18. A method of generating an imagecomprising the steps of: providing a quantum tunneling photodetectorarray comprising: an array of pairs of opposing first and secondelectrodes, the array of opposing first and second electrodes comprisinga cross-grid of a first set of electrode nano wires and a second set ofelectrode nano wires, such that the first and second electrodes areconstituted as opposing sections of the electrode nano wires of thefirst and second sets at projected cross-points of the cross-grid, aphoto-sensitive insulating material disposed between the opposing firstand second electrodes of the respective pairs, and an electrical circuitfor detecting photo-assisted quantum tunneling currents between theopposing first and second electrode of the respective pairs; detectingphoto-assisted quantum tunneling currents between opposing first andsecond electrodes of respective ones of the array of pairs of theopposing first and second electrodes; and generating the image based onthe respective detected photo-assisted tunneling currents between theopposing first and second electrodes.